In recent years, along with the development of FTTH, the opportunity for ordinary people other than a special construction dealer to touch an optical connector is increasing. In connector operations, when inserting and extracting the conventional SC type connector plug into and from a connector adapter, the connector plug remains in the connector adapter in the half-way in insertion or extraction, and it is known that a so-called half-way insertion occurs. In order to avoid this state, conventionally a marking was put on a part of the connector plug, and the connector plug was inserted or extracted so that the marking is invisible.